Submitted to: Applied Biochemistry and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 5, 2008
Publication Date: February 1, 2009
Citation: Jordan, D.B., Braker, J.D. 2009. Beta-D-xylosidase from Selenomonas ruminantium: Thermodynamics of Enzyme-catalyzed and Noncatalyzed Reactions. Applied Biochemistry and Biotechnology. 155(1-3):330-346. Interpretive Summary: Agricultural biomass like crop residues, grain processing byproducts, dedicated energy crops (e.g., switchgrass), etc., represent an abundant, renewable feedstock for production of ethanol and other valuable products if practical conversion technologies can be developed. These materials are rich in complex carbohydrates that must first be broken down to simple sugars that can be fermented by microorganisms to ethanol and other products. A critical step in the development of new conversion processes is the discovery and development of new enzymes to convert these complex materials to simple sugars. We have discovered an enzyme involved in the final step in the hydrolysis of xylan, the second most abundant carbohydrate in plants. This enzyme produces the simple sugar, xylose, more efficiently than counterpart enzymes described by other workers. The enzyme also catalyzes the release of arabinose from arabinosides. This work reveals temperature dependencies of the enzyme acting on natural and nitrophenyl substrates and compares them to spontaneous hydrolysis reactions occurring off the enzyme. Our results will help the development of new bioconversion strategies to produce fuel ethanol economically.
Technical Abstract: Beta-D-xylosidase/alpha-L-arabinofuranosidase from Selenomonas ruminantium (SXA) is the most active enzyme known for catalyzing hydrolysis of 1,4-beta-D-xylooligosaccharides to D-xylose. Temperature dependence for hydrolysis of 4-nitrophenyl-beta-D-xylopyranoside (4NPX), 4-nitrophenyl-alpha-L-arabinofuranoside (4NPA), and 1,4-beta-D-xylobiose (X2) was determined on and off (knon) the enzyme at pH 5.3, which lies in the pH-independent region for kcat and knon. Rate enhancements (kcat/knon) for 4NPX, 4NPA, and X2 are 4.3 x 10**11, 2.4 x 10**9, and 3.7 x 10**12, respectively, at 25 deg C and increase with decreasing temperature. Relative parameters kcat**4NPX/kcat**4NPA, kcat**4NPX/kcat**X2, and (kcat/Km)**4NPX/(kcat/Km)**X2 increase and (kcat/Km)**4NPX/(kcat/Km)**4NPA, (1/Km)**4NPX/(1/Km)**4NPA, and (1/Km)**4NPX/(1/Km)**X2 decrease with increasing temperature.